Abstract: Accounting for the classical statistical diffusion therory, a new formulation for the Lagrangian decorrelation time scale to be used in Lagrangian stochastic diffusion models has been derived. That is, expressions for the Lagrangian decorrelation time scales, depending on source distance, for inhomogeneous turbulence are proposed. The observed spectral properties and observed characteristics of energy-containing eddies are used to estimate these parameters. This decorrelation time scale is compared with their asymptotic limit and differences between them are discussed and analysed The major advanced shown in this paper is the necessity of including the downwind distance-dependent decorrelation time scale in air quality modeling.